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I've been reading all what you guys have been on saying pn underdriver pulleys and you've got me interested. I only have a couple questons about it now. Does it make your car actually go faster/feel a difference? Does it dramatiacally give you better gas mileage? Does it at all change the sound of the engine like make it louder? Does it add any horsepower? Thanks in advance for all your help.
 

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youngstangboy18 said:
I've been reading all what you guys have been on saying pn underdriver pulleys and you've got me interested. I only have a couple questons about it now. Does it make your car actually go faster/feel a difference? Does it dramatiacally give you better gas mileage? Does it at all change the sound of the engine like make it louder? Does it add any horsepower? Thanks in advance for all your help.
1) you will feel a difference past 2500 RPM especially, as the faster the engine turns, the more power the accesories rob from your system. When trying to pass someone on a 2 lane, you'll notice you can get around other cars much more quickly.

2) You won't necessarily fell a big difference in everyday city driving, but if you time 0-60, and especially 1/4 mile, you'll see a big difference, as MustangDewd did.

3) You'll see a 2-3 mpg gain on your gas mileage, especially at highway speeds for the reasons just described, AND it will extend the life of your accessories (alternator, power steering, water pump, AC compressor, etc.

4) It will NOT affect the sound of the engine, except to make it a bit less noisy as far as belt/pulley whine. Exhaust sounds are affected by the mufflers, not anything in the engine bay.

5) It doesn't "add" horsepower, it frees horsepower that's used to power your accessories. So, the hp at the crank is unchanged, but the hp at the WHEELS is increased.
 

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kscoyote said:
1) you will feel a difference past 2500 RPM especially, as the faster the engine turns, the more power the accesories rob from your system. When trying to pass someone on a 2 lane, you'll notice you can get around other cars much more quickly.

2) You won't necessarily fell a big difference in everyday city driving, but if you time 0-60, and especially 1/4 mile, you'll see a big difference, as MustangDewd did.

3) You'll see a 2-3 mpg gain on your gas mileage, especially at highway speeds for the reasons just described, AND it will extend the life of your accessories (alternator, power steering, water pump, AC compressor, etc.

4) It will NOT affect the sound of the engine, except to make it a bit less noisy as far as belt/pulley whine. Exhaust sounds are affected by the mufflers, not anything in the engine bay.

5) It doesn't "add" horsepower, it frees horsepower that's used to power your accessories. So, the hp at the crank is unchanged, but the hp at the WHEELS is increased.
everything you said is correct except for the part about HP at the crank. The HP at the crank will increase because you are making an adjustment to the motor. The only time you gain RW HP and not Crank HP is when you make an adjustment to the powertrain behind the flywheel (tranny on back). eg. Adding a cold air intake will increase HP at the flywheel and the RW's, but swapping out your 3.27's for 3.55's will add HP only at the RW's.
 

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ts3096 said:
everything you said is correct except for the part about HP at the crank. The HP at the crank will increase because you are making an adjustment to the motor. The only time you gain RW HP and not Crank HP is when you make an adjustment to the powertrain behind the flywheel (tranny on back). eg. Adding a cold air intake will increase HP at the flywheel and the RW's, but swapping out your 3.27's for 3.55's will add HP only at the RW's.
sorry, I still think in terms of 60's engine testing. You are correct, the accessories DO affect hp at the crank, SAE testing has changed 2x since the engines I worked on as a kid.
 

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ts3096 said:
everything you said is correct except for the part about HP at the crank. The HP at the crank will increase because you are making an adjustment to the motor. The only time you gain RW HP and not Crank HP is when you make an adjustment to the powertrain behind the flywheel (tranny on back). eg. Adding a cold air intake will increase HP at the flywheel and the RW's, but swapping out your 3.27's for 3.55's will add HP only at the RW's.
Close but no cigar. Swaping gears does NOT add Horse Power. It adds torque. A reduction gear, (as in a differential) is a torque multiplier. The only way to increase RWHP, not FWHP is to swap to a transmission that has less interal loss. Ie usually an auto for a manual, will show minimal gain.
 

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you are correct that it adds torque as the gear ratios act as a torque multiplier, but horse power is derived from torque, essentially torque over time (HP=torque * RPM / 5252). If you increase the torque, you increase the HP, and vice versa, but not neccesarilly you peak power. My statement was probably misleading.
 

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ts3096 said:
you are correct that it adds torque as the gear ratios act as a torque multiplier, but horse power is derived from torque, essentially torque over time (HP=torque * RPM / 5252). If you increase the torque, you increase the HP, and vice versa, but not neccesarilly you peak power. My statement was probably misleading.
just to be clear, it adds rwhp, as an effect of torque multiplication, not the hp from the engine.

I generally don't post EVERY facet of a mod -we'd be writing a book by the time the thread was done .. . .
 

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No, there is NO RWHP increase by changing rear gears. This is exactly the same as saying "my car has more HP in first gear than in second gear"
This is NOT the case.
If you put you car on a dyno, and run it in first you will get a certain HP reading. Then run it in second you will get the SAME reading.
Now put your car on the dyno with a 2.8 rear gear and take your HP reading, then swap the rear gear for a 3.75, your HP will be exactly the same and at exactly the same engine revs. It will show max HP at a lower road speed but it will be the same figure.
(there is potentially slightly different friction between different rear gears, but this would be so small as to be ignored)
 

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fast66 said:
No, there is NO RWHP increase by changing rear gears. This is exactly the same as saying "my car has more HP in first gear than in second gear"
This is NOT the case.
If you put you car on a dyno, and run it in first you will get a certain HP reading. Then run it in second you will get the SAME reading.
Now put your car on the dyno with a 2.8 rear gear and take your HP reading, then swap the rear gear for a 3.75, your HP will be exactly the same and at exactly the same engine revs. It will show max HP at a lower road speed but it will be the same figure.
(there is potentially slightly different friction between different rear gears, but this would be so small as to be ignored)
again, you are correct, the engine hp is the same.

but mathematically, if you measured the power at the wheels, using only torque to compute hp, the mathematical illusion is that there is more hp, as you're deriving hp from measured torque.

The dyno corrects this to measure engine hp. by using the RPM and engine measurements and computing dyno acceleration.

Does this make more sense?
 
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